Time-axis compression-expansion devices for sound signals

Abstract

The device of the present invention utilizes a processing principle for time-axis compression-expansion devices which presents variations of reproduction speed from reducing the intelligibility of sounds. Sound signals, without being subjected to any time-axis processing, are altered in frequency while some reduction in intelligibility would be expected to occur, the sound signals are subjected to time-axis transformation processing (compression-expansion processing) to yield reproduced signals of high intelligibility. The present invention is characterized by the capability of making this type of processing available through the use of simple, inexpensive system configurations such as an analog to digital conversion system based on delta/Δ modulation. The speed-changer signal processing circuitry is bypassed automatically when record-playback speeds are the same to avoid processing noise.

What is claimed is: 
     
       1. A time-axis compression-expansion device for use in reproducing an originally recorded signal stored in a recording medium, said device comprising: variable rate transducer means for reproducing said originally recorded sound signals at a frequency pitch and speed different from the pitch and speed used in recording and for producing a reproduced sound signal, said different frequency pitch adversely effecting the intelligibility of said reproduced sound signal;   time-axis compression-expansion processor means for changing the tone of said reproduced sound signal to correspond said tone to the tone of said originally recorded sound signal, thereby forming a converted sound signal including,   delta modulation means for converting said reproduced sound signal into a digital input signal; and   means for modifying said digital input signal to thereby form a converted digital signal; and   digital-to-analog converter means for changing said converted digital signal into an analog converted sound signal; and   means for converting said converted sound signal into an audible output having a frequency content similar to said originally recorded signal but having a speed different from the speed of said originally recorded signal;   said time-axis compression expansion processor means modifying said digital input signal so as to change the tone of said reproduced signal to correspond to the tone of said originally reproduced signal;   means for presenting said reproduced sound signal from said variable rate transducer means directly to said means for converting by bypassing said time-axis compression-expansion processor means when said speed of reproduction is substantially the same as said speed used in recording.   
     
     
       2. The device of claim 1, wherein said variable rate transducer means includes: a sound reproducing unit including a transducer for reading said signal stored on said recording medium; and   wherein said variable rate transducer means and said means for converting are located in a first housing;   said time-axis compression-expansion processor means, delta modulation means and digital-to-analog converter means being located in a second housing which is selectively connected to said first housing;   said variable time transducer means and said means for converting forming in combination a transducer for reproducing the audio waves corresponding to said originally recorded signal when said first and second housings are not connected;   time-axis compression-expansion processing being performed when said first and second housings are connected.   
     
     
       3. The device of claims 2 or 1, further comprising: a foot-plate pedal having a spring biased normal position corresponding to the original recording speed of said original recorded signal, the movement of said foot plate in a first direction speeding up the speed of said variable rate transducer means while the movement of said foot-plate in a second direction slows down the speed of said variable rate transducer means; and   means for varying the speed of said variable rate transducer means in response to the position of said pedal to vary the speed of said reproduced signal.   
     
     
       4. The device of claim 1, wherein said time-axis compression-expansion processor comprises: a random access memory;   means for converting said digital input signal into digital data and writing said digital data into said random access memory;   means for reading said digital data out of said memory at a different rate than said data was written into said memory to form said converted digital signal.